1. Field of the Invention
The field falls within electrical equipment for electricity distribution which comprises one or more electrical elements or components that correspond to one or more phases of the distribution network, housed in a container or tank, with the equipment insulated by means of a dielectric liquid wherein the elements are immersed.
2. Description of the Related Technology
The electrical equipment that forms part of electricity distribution networks, such as transformers and control and protection cells, are comprised of a series of elements or switchgear that are housed in a container, which is generally made of metal, inside which there is an electrical insulating means that completely surrounds the electrical equipment.
Basically, two types of alternative and clearly differentiated technologies have been developed, namely, technologies based on the use of a gas as an insulating means and technologies based on the use of a dielectric liquid (for example, oil) as an insulating means.
Of these two technologies, that which uses liquid as an insulating means is perhaps the oldest and it presents certain safety problems. For example, if there is an internal electric arc, said arc can cause all the equipment to explode, which involves not only the destruction of the equipment, but a serious hazard to anyone who is in the proximity of the equipment (since the explosion throws out both the different pieces or elements and the liquid—for example, oil—at a very high temperature).
In an attempt to solve or minimise these safety problems, the technology in which a gas (usually sulphur hexafluoride) was used as an electric insulating means was developed. The gas is contained in an airtight container, wherein the electrical equipment is positioned. This technology has been well developed and many engineers consider it to be safer and more modern than that which is based on the use of a liquid as an insulating means. In fact, in Europe this technology has almost completely substituted equipment using a dielectric liquid as an insulating means, except for the case of transformers (which continue to be housed in a tank full of dielectric liquid).
One of the tests to which the equipment with gas-based insulation is submitted is that known as the internal arc test, the purpose of which is to simulate the occurrence of a defect in the insulation between the phases. This test is performed by installing a thin metal conductor to join the phases and making the nominal voltage and current flow. Logically, the conductor melts, causing an electric arc to jump between the phases.
The manufacturers of cells insulated with gas as an insulating means have developed various different constructive solutions in order to pass this internal arc test. A first solution consists of arranging a series of metal sheets that can be sacrificed to the electric arc, so that the outer container of the cell is not damaged. The idea is, therefore, to try to minimise the effects of the electric arc rather than to prevent it.
Another second solution consists of using short-circuiting devices that join the phases of the electrical equipment by means of a conductor when the defect in the insulation is detected, so that the current is diverted through the area in which the short circuit, a so-called solid short circuit, has been established, thus preventing an arc from jumping between the phases. Devices and structures of this type are disclosed in, for example, FR-A-2687022, EP-A-1077518, EP-B-1052665, EP-A-1045415, EP-A-1005057, EP-B-0871190, EP-B-0795219, DE-A-4111586, DE-B-10254497, ES-T3-2126235 (Spanish translation of EP-B-0707364), WO-A-99/21255 and WO-A-00/62320, which relate to different short-circuiting devices for cells insulated with gas as an insulating means.
The short-circuiting device can act very quickly which means that most of the equipment in the cell can be saved and, in particular, any external manifestations that may put people and property at risk can be prevented. This short-circuiting device can also cause an external piece of protection equipment (e.g. the main switch for the distribution line) to open the line, completely insulating the cell. If an attempt is made to reconnect the line (closing the line switch again) without having resolved the fault that produced the internal arc, no damage is caused to the cell or the surrounding area because the short-circuiting device continues to work, preventing the arc from jumping between the phases.
One type of electrical equipment that plays a fundamental part in electricity distribution networks, and for which technology based on the use of a dielectric liquid as an insulating means continues to be widely used, is transformer equipment, which normally consists of a tank, which is practically full of dielectric liquid (normally mineral oil, although other liquids can be used, such as synthetic or natural esters, derived from plants, silicon oils or hydrocarbons with a high molecular mass, all of which can be with or without additives), wherein the transformer itself is located. If there is a fault in the insulation between the phases of the transformer, or between one of the phases and earth, there is a failure or internal arc that can generate a high pressure inside the tank, which could even cause the tank to explode.
For this reason, transformers are usually protected by medium voltage fuses that limit the current, which melt when a high current (produced by the internal arc) passes through them. Normally the fuse blows in a very short time, which means that it is possible to prevent the equipment from exploding.
These fuses are usually located outside the transformer tank but currently, in more compact solutions, the fuses can be positioned inside the tank; the term “self-protected transformers” is often used for this type of transformer equipment. Transformers of this type are disclosed in EP-A-1014528 and EP-A-0817346.
However, these self-protected transformers present the following problems:
When the current produced by a failure is small (for example, when the failure occurs between one of the phases and earth), the fuse can blow very slowly. In the case of the most commonly used limiting fuses, if the intensity of the failure current is lower than that corresponding to the minimum intensity of the cut-off current, the fuse partially blows but does not cut off the current, eventually leading it to explode. Therefore, the transformer is not protected against this type of failure. In order to resolve this problem microfuses are used, for example, which cause the phases to short circuit when they detect these failure currents of a lower intensity than that needed for the limiting fuses to open the corresponding line. U.S. Pat. No. 5,898,556 discloses a system of this type.
If there is a failure in the medium voltage fuse that limits the current and it explodes, an internal arc occurs in the tank transformer and the transformer is totally unprotected. No solution for this problem is known.
If the failure occurs before the fuses, i.e. between the input of the lines and the fuses, the fuses do not “pick up” the failure and therefore do not work. Again, the transformer is unprotected. No solution is known for this problem either.
Finally, if the failure occurs on the low voltage side, the intensity of the current that flows through the medium voltage fuses can be too low to blow the medium voltage fuses, or it could be the case that the fuses are partially blown but without cutting off the current, as has been described above. In any case, the low voltage failure can be enough to generate gases that raise the pressure inside the transformer tank and subsequently cause the equipment to explode.
Recent designs that attempt to prevent some of the aforementioned problems manage to protect the equipment by using automatic switches positioned inside the transformer tank next to the fuses. A transformer of this type is disclosed in EP-A-0981140A1.
This problem regarding transformers exists and current international regulations do not require transformers to pass an internal arc test similar to those mentioned for cells insulated in gas. This might be due to the fact that, in most cases, fuses are installed in protection cells outside the transformer tank; the explosion of a fuse in a cell would generate an arc in the cell, not in the transformer, and the cell is prepared to support this failure.
Therefore, the aforementioned problem occurs in electrical equipment insulated in dielectric liquid that is liable to suffer internal failures that generate gases such as, for example, transformers, self-protected transformers (i.e. transformers that have their protection fuses inside the tank), which can also be a more compact and cheaper solution than a transformer and its protection cell with fuses, meaning that the use of this type of transformers is becoming more common for some applications.
As well as the transformer, not only the fuses but also the sectionalising switches and other types of elements or switchgear can be inserted into the same tank, thus obtaining a transformer substation that can be used for control and/or protection that comprises a container or metal tank with medium or high voltage phase bushings and low voltage outlet terminals, with all the equipment or elements immersed in the dielectric liquid contained in the tank. Including all these elements inside a single tank means that the volume of the dielectric liquid used increases.
This presents two problems:
If the container or tank breaks (e.g. due to an impact from outside the tank) a large volume of oil is spilt, which can also burn, with the resulting consequences. This problem can be lessened by covering the whole metal tank with an outer concrete housing and a dielectric collection pit.
In the event of there being an internal arc, the transformer substation may explode; the same problems exist as in the case of self-protected transformers, but they are made worse by the greater volume of dielectric liquid.
The presence of moving parts, together with the cut off of intensity in the liquid, increases the probability of an accident occurring in the equipment.